NaPTAN is a GB national system for uniquely identifying all the points of access to public transport in GB. It is a core component of the GB national transport information infrastructure and is used by a number of other UK standards and information systems. Every GB station, coach terminus, airport, ferry terminal, bus stop, etc., is allocated at least one identifier.

Relevant links:
NaPTAN home page on DfT website (for background information)
National Public Transport Gazetteer
National Public Transport Data Repository (public transport timetables)
Traveline National Data Set (a weekly snapshot of public transport timetables due to be produced from early / mid 2012)